1. Field of the Invention
The present invention relates to an oxygen-type bleaching composition. More particularly, the invention relates to a bleaching composition which can be used to bleach even a colored or patterned fabric which has been dyed with a metal-containing dye or pigment.
2. Description of the Prior Art
In general, an oxidation reaction is utilized for bleaching operations conducted both in households and industry. Chlorine-type bleaching agents are mainly used for this bleaching reaction. However, many colored and patterned fabrics cannot safely be bleached by these chlorine-type bleaching agents. Accordingly, oxygen-type bleaching agents capable of bleaching even colored and patterned fabrics have recently been developed and used. However, it has been found that the safety of these oxygen-type bleaching agents is insufficient with respect to the bleaching of colored and patterned fabrics dyed with metal-containing dyes or pigments or treated with metal salts or metal-containing fixing agents.
There are known methods in which aminocarboxylic acids and their water-soluble salts such as NTA (nitrilotriacetic acid), EDTA (ethylenediamine tetraacetate) and DTPA (diethylenetriamine pentaacetate), or inorganic salts such as polyphosphates, magnesium salts and silicates are incorporated into oxygen-type bleaching agents so as to stabilize the peroxides in bleaching solutions, to prevent degradation of fluorescent whitening agents and to prevent the fibers of which the fabric is made from becoming brittle. However, when colored and patterned fabrics dyed with metal-containing dyes or pigments or treated with metal salts or metal-containing fixing agents are bleached according to these known methods, there are brought about various disadvantages. For example, no substantial effect can be attained if such additives are not incorporated in very large amounts, or the color tone or hue is changed or degraded in the treated fabrics.
More specifically, a metal chelating agent of the amino acid type such as NTA, EDTA or DTPA has no substantial effect if it is not incorporated in a very large amount. A polyphosphate such as sodium tripolyphosphate, sodium pyrophosphate or sodium hexametaphosphate or a hydroxycarboxylic acid salt, such as sodium citrate, does not exhibit any of the foregoing desired effects at all. Further, a magnesium salt, such as magnesium silicate, rather promotes decoloration and discoloration of colored and patterned fabrics.
The mechanism of decoloration or discoloration in colored and patterned fabrics will now be considered.
These colored and patterned fabrics are dyed by the dipping or printing method. For example, when a woven fabric is dyed with a metal-containing direct dye or a metal-free direct dye, the dyed fabric is ordinarily treated with a metal-free fixing agent or a metal-containing fixing agent or a metal salt so as to improve the swelling or light resistance of the treated fabric. When a woven cotton fabric is dyed according to the printing method, in order to obtain a good swelling resistance and a clear color, a reactive dye containing a metal in the dye molecule is mainly used. Copper, cobalt, chromium and nickel are used as such metal, and the use of copper is most popular. The metal in a colored or patterned fabric dyed according to such method catalytically reacts with hydrogen peroxide in an aqueous bleaching bath to form a hydroxyl radical from hydrogen peroxide, and it is considered that this hydroxyl radical causes discoloration or decoloration of colored and patterned fabrics. This undesirable phenomenon is especially conspicuous when the metal is copper. Therefore, in order to prevent discoloration or decoloration in such colored or patterned fabrics, it is very important to find a chelating agent which is capable of deactivating the metal in the colored or patterned fabric, i.e., completely filling the coordination sites of the metal, so that the metal cannot act as a catalyst for decomposing hydrogen peroxide. The chelating agent as well as metal chelate compounds thereof also should be non-oxidizing and should be capable of coordination with a metal which has already been chelated with the dye.